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  1. 1. M.S.VENKATA RAMAYYA* LOLLA SUDHAKAR** M.V.RAMANA RAO*** PREDICTION OF SURFACE SUBSIDENCE WITH INTERACTION OF GOAF (PRESS WING) ABSTRACT : India has a long history of commercial coal mining covering nearly 220 years starting from 1774 by M/s Sumner and Heatly of East India Company in the Raniganj Coal field along the western bank of river Damodar. The Singareni Collieries Company Limited (SCCL) is one of the biggest public sector companies in India fulfilling the coal need of South India since 1945. The different technologies are adopted for extracting coal from underground dealing with many problems. One of the major problems in every technology is ‘Subsidence’. Different scientific agencies suggested different formulae for predicting maximum subsidence for different regions of SCCL. For Kothagudem region of SCCL None of them explained the interaction of goaf in different seams. For Kothagudem region of SCCL, a number of panels including Bord & Pillar and Longwall have been studied for correct estimation of surface subsidence. This paper deals with the procedures for prediction of subsidence with interaction of goaf in different seams. The estimate is made from the field data after thorough study of strata properties and the conditions existing here. Padmavathikhani (PVK) project has worked 9 longwall panels of which 8 have been completed. These panels were worked in upper seam after the depillaring of lower seams. The partition between upper and lower seams is 45m. Normally, the system of extraction by caving is by descending order, where as in this mine it has been done in the ascending order. The system of extraction in the lower seam was predominantly by Bord and Pillar with caved, stowed goaves in 2 to 3 sections. The system of subsidence prediction by actual measurements in the field and co-relation to the established theories was made in this project. A number of discrepancies have been observed in the analysis, which were not co-related to the established theories. Since Subsidence depends on different factors like width of extraction, depth, strata properties, angle of draw, time… etc. An attempt has been made in this paper to understand the actual behaviour of the strata in relation to the extracted area and the actual amount of Subsidence. The analysis made is found to be site specific to this project in the geo-mining conditions of PVK. Similar studies can be conducted based on the above theory to analyze subsidence in multi-section and multi-seam workings with different technologies. * Dy.GM (5Group of Mines) ** Dy.Mgr (Project Planning) *** Under Manager, No.5 Incline The Singareni Collieries Company Limited, Kothagudem – 507 119, Andhra Pradesh, INDIA.
  2. 2. INTRODUCTION : Padmavathi Khani (PVK) mine is the part of No.5 Incline, opened mainly for coal extraction by Long wall method in Top seam of 9.75m thickness. Till now 8 Long wall panels are extracted successfully with 150m face length and 9th panel with 60m face length is under extraction, the face length is reduced from 150m to 60m due to shallow depth of the panel. (worked out longwall panels and presently working panel is shown in PLAN-1). Since from the beginning of the PVK project, lot of study has been conducted by the mine management and different scientific agencies in predicting the surface subsidence. NIRM has studied extensively and arrived at some empirical relation to calculate the maximum subsidence value for different regions of SCCL. But the predicted subsidence profile using NIRM formula for Kothagudem region is giving different results than the actual, is due to the existence of caved and stowed goaves of King seam beneath the longwall panels of Top seam. The observed surface profiles of worked out panels (shown in Annexure-I) clearly indicating that the subsidence profiles are asymmetrical to their central axis as the angle of draw and subsidence are generally more on the starting side of the panel than the finishing side. This is because of the fact that the energy released in the first break will be higher than the energy released in the subsequent break1. Also the subsidence value is more than the normal, whenever any goaf is encountered in below seam beneath the panel. This paper places the subsidence prediction process in proper perspective by through study of already obtained profiles of worked out panels and arrives at an empirical formula to predict the maximum subsidence. In this paper, an attempt has made to formulate the effect of goaf on subsidence for this particular geo-mining condition of Kothagudem area. From this curve, the proper correction factor is to be applied to the predicted value of subsidence depending on the existence of percentage of goaf beneath the panel to obtain the correct value. SUBSIDENCE PREDICTION TECHNIQUES: Existing subsidence prediction techniques fall under two basic categories. • Empirical methods • Phenomenological methods The empirical theories are principally based on observations and experience from field subsidence studies. Some of the empirical methods have proved sufficiently reliable for subsidence prediction, at least for a given region. Many of these have been successfully applied in a number of countries, especially in Europe. The empirical profile function method involves the derivation of a mathematical function that can be fitted to plot a complete profile. The constants employed in the profile function are empirically derived from observed data. Once a function is established through the use of actual field data, it can be used to predict subsidence profiles over future areas of mining. Profile functions have been successfully used in Hungary, Poland and Russia.
  3. 3. Phenomenological techniques are based on equivalent material modeling principles where the subsiding strata are mathematically represented as idealized materials that obey the laws of continuum mechanics. Unlike empirical methods, the procedures used in the latter category have not achieved much success to date, mainly due to the difficulty of representing complex geological properties of the strata in simple mathematical terms. PREDICTION OF MAXIMUM SUBSIDENCE - NIRM: After collection and study of subsidence data from 111 previous worked out panels (both longwall and bord & pillar panels) of Kothagudem region, NIRM scientists2 has suggested a Non-Linear equation relating the width to depth ratio (W/H) and subsidence factor (Maximum subsidence/ height of extraction) of the following type. S = h x a /(1+ ((W/H)/b)-c) Where, S = Maximum subsidence, m h = Effective height of Extraction ( height of extraction x % of extraction), m W = Width of the panel, m H = Depth of the panel, m a, b, c = Constants, The values for Kothagudem region are given as a = 0.66 b = 0.8 c=7 Therefore, S = h x 0.66 /(1+ ((W/H)/0.8)-7) The results obtained by applying the above formula to the already worked out longwall panels at different W/H ratios are showing different profiles than the actual profiles (predicted and actual profiles in detail form is given in Annexure-II). The starting part of the profile is somewhat co-in siding with the actual profile but there onwards it is showing incorrect value and at the end it is not closing with the ground profile. PREDICTION MODEL : (for Kothagudem Area) Methodology followed in arriving an empirical formula for prediction of maximum subsidence is given below. 1. Surface subsidence profiles of different worked out longwall panels are drawn from the survey results conducted on the surface over them. 2. For each profile the subsidence values are co-related with different W/H ratios of that particular panel by marking W/H ratio along the horizontal axis. 3. The polynomial equation with the relation between W/H ratio and subsidence for each of the subsidence profile is obtained by using the computer.
  4. 4. 4. The combined statistical average of all the polynomial equations is calculated to arrive at a final equation. 5. Thus obtained equation gives the relation between subsidence and W/H ratio and can be used for predicting the maximum subsidence. By following the above mentioned method, the following equation is obtained, which is specific to Kothagudem area, because it covers the geological conditions of this area only. S = 0.1508 (W/H)2-0.8248(W/H)-0.5292 ---------- (Eq. 1) Where, S = Maximum subsidence, m W = Width of the panel, m H = Depth of the panel, m The subsidence profiles obtained for different panels using the above formula (shown in Annexure-III) are more or less co-in siding with the actual profiles, but at some points the variation is more and it is observed from the working plan of the mine, the difference is due to the presence of goaves in King seam beneath the longwall panels of Topseam. INTRODUCING THE GOAF FACTOR : The percentage of goaf area present beneath each sub part of the panel or between the two consecutive subsidence stations present on the surface is calculated as shown in above figure. The variation between the predicted subsidence (using Eq.1) and the actual subsidence of worked out panels is co-related with the percentage of goaf areas present beneath the panels and arrived at a solution (Variance Vs Goaf percentage curve is shown in Annexure-IV) given below.
  5. 5. Gf = 0.0001(P)2-0.0128(P)-0.3265 --------- (Eq. 2) Where, Gf = Goaf Factor. P = Percentage of Goaf area. After calculating the goaf factor, just add it to the predicted subsidence (using Eq.1) to get the actual subsidence, which almost co-in sides with the original subsidence value. Thus, Subsidence = Eq.1 +Eq.2 Subsidence = [0.1508(W/H)2-0.8248(W/H)-0.5292]+[0.0001(P)2-0.0128(P)-0.3265] In case of a stowed goaf, add 1-(P/100) to the final subsidence. The corrected profiles after applying the goaf factor and the original profiles of worked out panels are given in Annexure-V. ACCURACY / VALIDITY: Since the predictions are made based on a vast data available with wide variations, it is not possible to accurately predict any value, but can be predicted with in the permissible limits. Mining is such an activity that is affected by different factors and these factors takes part in accuracy also. Some of the factors affecting subsidence prediction are given below. Geological factors - Geological disturbances, Faults, folds, dykes, Topography, surface cover, … Mining factors - Seam thickness, depth, dip, Extraction width, Goaves, Rate of extraction, Technology… Strata factors - Physical properties, bedding, jointing, Seam roof and floor condition, nature, … In Britain, a maximum variation of plus or minus 10 percent was considered acceptable in subsidence3 and in many cases a high degree of accuracy is not required. If any important structure is to be protected strictly, then the need of accuracy arises, which is very rare in mining. Comparison statement of the predicted and actual subsidence for the worked out panels is given below. Panel Max. Subsidence (mm) Actual Predicted Diff. % Panel no.2 2545 2385 +160 +7 Panel no.3 1930 2221 -291 -13 Panel no.4 1790 2112 -322 -15 Panel no.5 2196 2265 -69 -3
  6. 6. The difference between the actual and the predicted subsidence varied between 3% and 15%. As the negative side of subsidence is more, indicating that the predicted value is more than the actual and covers the other possible dangers due to subsidence. CONCLUSION: Prediction of surface subsidence, finds its use mainly for • The planning of surface land use in coal bearing areas in cases when surface development precedes mining. • The design of mine layouts for mining coal from underneath existing surface developments. As these curves are site specific, they cannot be applied to the other regions of coal mining having different geo-mining conditions, but the similar study can be done with the available large data and empirical formulae can be developed for predicting the surface profiles above the future working panels. ACKNOWLEDGEMENT: The authors are grateful to the Singareni Management in particular and Sri P Vasudeva Rao Dir (Opr) and Sri CH Krishiah GM, KGM for encouraging in bringing out this paper. The views expressed by the authors are of their own and not necessarily of the organization. REFERENCES: 1. Dhar B.B. (1995), Status of subsidence research in India, Acourse on Subsidence prediction and management in mining areas by CMRI, 18 th-22nd November, 1995.,pp.1-11. 2. NIRM Report, “Subsidence studies at SCCL”. 3. Orchard R.J., (1964), Surface subsidence resulting from alternate treatment of colliery goaf, Colliery Engineering, October, pp.428-435. 4. Holla L., (1991), Reliability of Subsidence prediction methods for use in Mining Decisions in New South Wales.
  7. 7. PLAN – 1
  8. 8. ANNEXURE –I OBSERVED SUBSIDENCE PROFILES OF DIFFERENT LONGWALL PANELS Panel no.2 0 6 5 4 W/H 3 2 1 0 -0.5 -1 Subsidence (m) -1.5 -2 -2.5 -3 Panel no. 3 0 6 5 4 3 2 1 0 W/H -0.5 Subsidence (m) -1 -1.5 -2 -2.5
  9. 9. OBSERVED SUBSIDENCE PROFILES OF DIFFERENT LONGWALL PANELS Panel no.4 0 5 4 3 2 1 0 -0.2 W/H -0.4 -0.6 Subsidence (m) -0.8 -1 -1.2 -1.4 -1.6 -1.8 -2 Panel no.5 0 6 5 4 3 2 1 0 W/H -0.5 Subsidence (m) -1 -1.5 -2 -2.5 ANNEXURE-II
  10. 10. OBSERVED SUBSIDENCE Vs PREDICTED SUBSIDENCE WITH NIRM FORMULA PANEL NO.2 0 7 6 5 4 W/H 3 2 1 0 -0.5 -1 Subsidence (m) -1.5 -2 -2.5 NIRM-Formula Actual Subsidence -3 ANNEXURE-III
  11. 11. OBSERVED SUBSIDENCE Vs PREDICTED SUBSIDENCE WITH EQ.1 S = 0.1508 (W/H)2-0.8248(W/H)-0.5292 ---------- (Eq. 1) Panel no.2 0 6 5 4 W/H 3 2 1 0 -0.5 -1 Subsidence (m) -1.5 -2 act.sub -2.5 Pr.Sub -3 Panel no. 3 0 6 5 4 3 W/H 2 1 0 -0.5 Subsidence (m) -1 -1.5 act.sub -2 pr.sub -2.5 OBSERVED SUBSIDENCE Vs PREDICTED SUBSIDENCE WITH EQ.1
  12. 12. Panel no.4 0 5 4 3 2 1 0 -0.2 W/H -0.4 -0.6 Subsidence (m) -0.8 -1 -1.2 -1.4 -1.6 act.sub -1.8 pr.sub -2 Panel no.5 0 6 5 4 3 W/H 2 1 0 -0.5 Subsidence (m) -1 -1.5 -2 act.sub -2.5 pr.sub ANNEXURE-IV
  13. 13. CURVE FOR FINDING THE GOAF FACTOR 0.4 0.2 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 goaf% Goaf Factor -0.2 y = 0.0001x2 - 0.0128x - 0.3265 -0.4 -0.6 Goaf.fact Poly. (Goaf.fact) -0.8 ANNEXURE-V
  14. 14. OBSERVED SUBSIDENCE Vs PREDICTED SUBSIDENCE AFTER APPLYING GOAF FACTOR Panel no.2 0 6 5 4 W/H 3 2 1 0 -0.5 -1 Subsidence (m) -1.5 -2 -2.5 act-sub pred-sub -3 Panel no. 3 0 6 5 4 3 2 1 0 W/H -0.5 -1 Subsidence (m) -1.5 -2 -2.5 p3 pred -3 OBSERVED SUBSIDENCE Vs PREDICTED SUBSIDENCE AFTER APPLYING GOAF FACTOR
  15. 15. Panel no.4 0 5 4 3 2 1 0 W/H -0.5 Subsidence (m) -1 -1.5 -2 p4 pred -2.5 Panel no.5 0 6 5 4 3 2 1 0 W/H -0.5 Subsidence (m) -1 -1.5 p5 -2 pred -2.5